Losses in a microwave transmission line establish a limit on the maximum distance that a signal will be allowed to propagate before it has been attenuated to the point of existing with undesirably low signal-to-noise ratio. Losses in a resonator or filter circuit limit the frequency discrimination that can be effected with such components. It is therefore generally desirable to construct microwave circuits that have a minimum amount of loss.
The sources of loss in a microwave structure are radiation loss, conductor loss, and dielectric loss. Radiation loss may be minimized by shielding of a circuit, i.e., putting it in a closed metal container. Conductor losses can often be minimized by using superconducting materials which are operated appreciably below their critical temperature, T.sub.c. Dielectric loss, which is due to the imperfect behavior of bound charges, exists whenever dielectric materials are located in a time varying electric field.
Recently, strip-type microwave superconducting transmission lines that utilize the "kinetic inductance" of superconductors have been fabricated. It has been demonstrated that these lines can propagate microwave energy at speeds on the order of 0.01 c, where c is the speed of light in free space. See "Measurements and Modeling of Kinetic Inductance Microstrip Delay Lines", IEEE Trans. on Microwave Theory and Tech., MTT-35, no. 12, pp. 1256-1262, December 1987, by J. M. Pond, J. H. Claassen, and W. L. Carter. The basic structure of these lines is shown in FIG. 1. As indicated in FIG. 1, such lines were fabricated by depositing a "ground plane" 12 of very thin superconducting material on an appropriate substrate for thermal bonding techniques. This deposition was followed by a very thin dielectric layer 14. Another very thin superconducting film 16 was deposited on top of this structure and patterned, so as to produce the microstrip structure as shown in FIG. 1.
Such a superconducting transmission line has a propagation velocity, V.sub.p, given by: EQU v.sub.p =(LC).sup.-1/2
where L is the inductance per unit length and C is the capacitance per unit length. For situations where the three thicknesses of layers 12, 14 and 16 are all much smaller than the superconducting penetration depth, .lambda., the inductance is determined by the kinetic inductance, which is orders of magnitude greater than the magnetic inductance. It is under these conditions that phase velocities of 0.01 c are obtainable. A criterion for the "kinetic inductance" to be dominant is that: EQU .lambda..sub.i coth(t.sub.i /.lambda..sub.i)&gt;d , i=1 or 2
where d is the thickness of the dielectric and t.sub.1 and t.sub.2 are the thicknesses of the thin film superconductors as shown in FIG. 1, and .lambda..sub.1 and .lambda..sub.2 are the corresponding penetration depths of the superconducting films 12 and 16. Since the effective wavelength, .LAMBDA., of the propagating wave in such a transmission line is given by: EQU .LAMBDA.=v.sub.p /f,
where f is the frequency, a half wavelength resonator at 3 GHz with v.sub.p =0.01 c is only 0.5 mm long, whereas a half-wavelength resonator for an ordinary strip line with a dielectric of relative dielectric constant .epsilon..sub.r =2.3 would be 3.3 cm. in length.
Similarly, to delay a microwave pulse by 100 ns would require an ordinary strip line with .epsilon..sub.r =2.3 to have a length of 20 m, whereas the superconducting delay line with v.sub.p =0.01 c would require length of only 30 cm. Since the width of the superconducting line is on the order of 20 .mu.m as demonstrated in the above reference ("Measurements and Modeling of Kinetic Inductance Microstrip Delay Lines"), such a line could be fabricated very compactly in a spiral or meander pattern.
The attenuation of this line has been found to be dominated by dielectric losses and hence the loss is given by: EQU .alpha..sub.d =(.pi.f/v.sub.p)(.epsilon."/.epsilon.').
where .epsilon."/.epsilon.' is the dielectric loss tangent. With a dielectric loss tangent of 10.sup.-3, the loss of this line for 3 GHz signals would be 8.2 dB. The Q (which determines the frequency selectivity) of the 3 GHz superconducting resonator mentioned above is given by: EQU Q=.epsilon.'/.epsilon."=1000.
It is seen that the attenuation of a delay line and the Q of a resonant structure of a compact superconducting structure of the type shown in FIG. 1 are limited by the dielectric material.
Clearly, the most desirable situation is to use a dielectric of vanishingly small loss tangent. Such an ideal dielectric is vacuum or gas (e.g. argon, nitrogen, etc.). Previously there has not been a method of fabricating the microstrip structures which had a small enough dielectric thickness (gap) between conductors, which dielectric thickness was also substantially uniform and low loss over a large enough area to produce a device of any practical use.